Many processes use heated rolls in one or more steps of the process. Some require a high degree of heat uniformity on the roll surface, to achieve uniformly acceptable results. As heat is removed from the roll, heat distribution can become transiently non-uniform, and heating the roll preferable will account for the non-uniformity that occurs during use.
For example, printers, copiers and other printing devices commonly use an electrophotographic printing process in which a uniform charge is applied to a photoconductive surface on a drum or belt. A light beam, such as from a laser, is used to expose the surface, leaving an electrostatic latent image corresponding to the image to be printed. The latent image is developed by the application of toner particles that adhere to the electrostatic latent image. The toner image is transferred to the media intended to receive the image and is fixed thereon through the application of heat and or pressure in a fuser.
The fuser commonly includes opposed cooperating rollers, one of which is heated and is commonly referred to as a fuser roll. A pressure roll is nipped there against, and the media bearing the toner image passes between the opposed rolls wherein heat and pressure is applied to fix the toner particles on the media.
For high quality printed images, heat and pressure applied must be consistent and uniform. As each piece of media passes through the fuser, heat is transferred from the fuser roll to the media in the area at which the media contacts the roll. The surface temperatures become non-uniform, and, before a next piece of media passes through the fuser, the fuser roll must be reheated. If printing on one size medium is followed immediately by printing of a different size medium, the second medium may receive non-uniform heat applied thereto from the fuser roll unless the reheating process accounts for the non-uniformity of surface temperature following the preceding fusing process. Since proper fusing of fusible inks is a function of heat residence time and pressure, if the pressure and residence time are the same but the heat applied varies from one part of the medium to another part of the medium, non-uniform fusing can occur. Under-fused inks can cause smearing, offset and other unacceptable conditions.
As heat is applied, non-uniformity can remain if all areas are heated equally, until a maximum temperature is reached and distributed evenly. A simple solution to the non-uniformity of heat distribution is to allow a sufficient time lag between fusing operations for the fuser roll to reheat and equalize. However such a delay itself can be unacceptable. In photocopiers and printers, increased speed and performance are highly sought after. Thus, delaying a subsequent printing function for a different size medium following a first size medium can require unacceptable delay in a high-speed office machine. It is known to use heat pipes to achieve improved heat distribution and uniformity. However, the heat response time of a heat pipe depends on the outer surface material, and it is often difficult to maintain a very thin outer surface on a heat pipe. The heat pipe requires a liquid or vapor within the heat pipe, and the assembly is somewhat cumbersome to install and expensive to manufacture.
It has been found in fuser rolls of electrophotographic processes that the heat response time in the radial direction is relatively short and the heat transfer rate in the axial direction of the roll is relatively slow. The difference in surface reheating can be even slower when TEFLON® material or other coatings have been applied to the fuser roll as a release agent.
What is needed in the art is an improved heated roll that transfers heat from a hotter portion thereof to a colder portion thereof.